Cup grinding wheel

ABSTRACT

A cup grinding wheel  20  which is mounted on a rotating tool  10  and rotates to grind a surface of a physical object is formed of a central base member  21   a  approximately shaped like a circular disc having a mounting hole  22   a  at a center thereof, an exterior base member  21   b,  a grinding portion  27  and a screw member  29.  The exterior base member  21   b  is formed of a rim engaging portion  25  which can be engaged with an outer circumferential portion of the central base member  21   a,  and a flanged rim fastening portion  26  provided on a front rim of the rim engaging portion  25.  Furthermore, the grinding portion  27  is fixed to the front of the rim fastening portion  26  of the exterior base member  21   b  with the screw member  29.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cup grinding wheel which is mounted on a rotating tool and rotates to grind a surface of a physical object.

2. Description of the Related Art

For grinding a surface of a physical object such as a surface of a concrete wall to make the surface flat, a rotating tool having a cup grinding wheel shaped approximately like a circular disc is conventionally used. More specifically, the cup grinding wheel shaped like a circular disc having a mounting hole at the center of the disc has curved surfaces configured such that one surface around the mounting hole is convex, and the other surface around the mounting hole is concave. In general, the cup grinding wheel is configured such that grinding chips including diamond abrasive grain are fixed by welding on the circumference of a concave surface of a metal base plate. However, this cup grinding wheel produces significant vibration and noise during the use of the cup grinding wheel for grinding, presenting a problem that the vibration and noise annoy a user of the cup grinding wheel and residences living in the neighborhood of the workplace. In order to cope with this problem, therefore, a cup grinding wheel which has a base plate made of a resin material and has an elastic sheet between the base plate and grinding chips to reduce vibration and noise is also used (see Patent Literature 1, for example).

This cup grinding wheel (disc-shaped grinding stone) is formed of an approximately ring-shaped base plate made of a resin material, an elastic sheet adhered to the base plate by adhesive, a holding plate overlaid on the elastic sheet and adhered to the elastic sheet by adhesive, and grinding chips (abrasive chips) fastened to the holding plate. The grinding chip is formed of an abrasive grain layer and a metal piece having a swaging portion, while the holding plate is provided with a through hole into which the swaging portion can be inserted. By inserting the swaging portion into the through hole to swage the end of the swaging portion, the metal piece is integrated with the holding plate. By employing the elastic sheet, this conventional cup grinding wheel suppresses vibration to prevent the grinding chips from falling away from the base plate. By using a resin material for the base plate, furthermore, vibration and noise can be reduced.

CITATION LIST Patent Literature

[PTL 1] JP 3762754

SUMMARY OF THE INVENTION

However, since the above-described cup grinding wheel is configured such that the base plate and the elastic sheet, and the elastic sheet and the holding plate are adhered by adhesive, the base plate and the elastic sheet, and the elastic sheet and the holding plate are apt to fall away from each other. In general, adhesive property of an adhesive reduces in a high-temperature condition of about 100 degrees Celsius. Therefore, if the cup grinding wheel is heated to a high temperature by friction heat for grinding a physical object with a rotating tool, the adhesion of the adhesive is reduced. Particularly, in a case where grinding chips are worn to get thin from use, heat generated on the grinding chips is easily conveyed to the adhesive which adheres the holding plate to the elastic sheet, so that the holding plate is apt to fall away from the elastic sheet.

In order to firmly adhere the base plate to the elastic sheet, and the elastic sheet to the holding plate, furthermore, it is necessary to broaden adhesive areas. Consequently, it is necessary to upsize the members adhered by an adhesive, which causes problems that the weight of the cup grinding wheel is increased, and that the cup grinding wheel is upsized. Furthermore, the above-described conventional cup grinding wheel presents a problem that the number of parts is increased due to the use of elastic sheet, so that the number of manufacturing processes is increased.

The present invention was accomplished to solve the above-described problems, and an object thereof is to provide a cup grinding wheel which reduces vibration and noise produced by grinding, prevents a grinding portion from falling away, achieves weight reduction, and facilitates manufacturing. In descriptions of constituent features of the present invention which will be described below, numerical references of corresponding components of embodiments are given in parentheses in order to facilitate the understanding of the invention. However, it should not be understood that the constituent features of the invention are limited to the corresponding components of the embodiments indicated by the numerical references.

In order to achieve the above-described object, it is a feature in configuration of the cup grinding wheel according to the present invention to provide a cup grinding wheel (20) which is mounted on a rotating tool (10) and rotates to grind a surface of a physical object, the cup grinding wheel including a central base member (21 a) approximately shaped like a circular disc having a mounting hole (22 a) at a center thereof; an exterior base member (21 b) formed of a rim engaging portion (25) which can be engaged with a circumferential portion of the central base member, and a flanged rim fastening portion (26) provided on one rim of the rim engaging portion; a grinding portion (27, 37, 47); and a screw member (29) for fastening the grinding portion to one surface of the rim fastening portion so that the grinding portion can be detached.

The cup grinding wheel according to the present invention is configured such that a base plate portion which makes up a main body of the cup grinding wheel is formed of the central base member and the exterior base member which are separated from each other, with the grinding portion being fastened to the rim fastening portion of the exterior base member with the screw member. Because of this configuration, even though vibrations caused by grinding are transferred from the grinding portion to the exterior base member when the cup grinding wheel is rotated with the grinding portion being pressed against a physical object, the central base member serves as a vibration isolator or an interrupter which interrupts transmission of vibrations to reduce vibrations of the exterior base member or to interrupt vibrations of the exterior base member to prevent the vibrations from growing. As a result, vibrations of the entire cup grinding wheel are suppressed to the same level of vibrations that are generated in the exterior base member or less. Consequently, vibrations generated in the entire cup grinding wheel are significantly reduced, so that noise can be also significantly reduced.

Furthermore, the grinding portion is fastened to the rim fastening portion of the exterior base member with the screw member. Therefore, the possibility that the grinding portion is released from the exterior base member due to heat can be eliminated. Furthermore, this configuration can reduce an area where the grinding portion is in contact with the rim fastening portion, so that the grinding portion can be minimized to achieve reduction of the weight of the cup grinding wheel. In addition, fastening of the grinding portion to the exterior base member is facilitated, so that the manufacture of the cup grinding wheel is also facilitated. Furthermore, the exchange of the exhausted grinding portion is also facilitated.

It is preferable that the engagement between the central base member and the rim engaging portion of the exterior base member is thread engagement. More specifically, the direction in which the cup grinding wheel rotates in order to grind a physical object coincides with the direction in which the thread engagement between the central base member and the exterior base member is fastened. Furthermore, the central base member may be configured to have a tapered part in which one surface is convex and the other surface is concave, and may be configured such that a peripheral wall is provided on the rim of a circular disc. As for the present invention, furthermore, the “one” rim of the rim engaging portion and the “one” surface of the rim fastening portion indicate an identical direction. For example, on the understanding that the direction of thickness of the cup grinding wheel is defined as the front-rear direction, if the one rim of the rim engaging portion is a front rim, the one surface of the rim fastening portion is a front surface. Furthermore, the screw member includes various kinds of screws and bolts.

It is another feature in configuration of the cup grinding wheel according to the invention that a plurality of ejecting holes (23 a) are provided around the mounting hole of the central base member.

In this invention, because the central base member is provided with the ejecting holes, shavings ground away from a physical object can be ejected from a ground surface to the opposite side of the cup grinding wheel. As a result, the cup grinding wheel according to the invention ensures smooth grinding work, eliminating occasions where shavings are accumulated on the ground surface to hinder the grinding work. The ejecting holes are effective particularly in a case where the cup grinding wheel is mounted on a rotating tool having a mechanism of ejecting shavings. Furthermore, the ejecting holes can also help the central base member prevent vibrations and reduce noise. Furthermore, the ejecting holes provided on the central base member can contribute to weight reduction of the cup grinding wheel and noise reduction.

It is still another feature in configuration of the cup grinding wheel according to the invention is that the grinding portion is configured such that a grinding chip (28) or grinding abrasive grain is fixed on one surface of a supporting base plate (27 a, 37 a, 47 a) made of iron or steel, and the supporting base plate is fastened to the rim fastening portion.

In this case, it is preferable that a sintered product obtained by molding and sintering diamond abrasive grain by use of bonding agent is used as the grinding chip. More specifically, the grinding portion is manufactured by brazing, such as silver brazing, and fastening the sintered product to the supporting base plate. This configuration can eliminate the need for the grinding chip to be provided with holes for inserting screw members. Therefore, this configuration can facilitate molding of the grinding chip, also eliminating useless space unavailable for grinding from the grinding chip.

The grinding portion of this case can be configured by fixing a plurality of grinding chips at certain intervals on a ring-shaped supporting base plate, or by arranging a plurality of supporting base plates to each of which a grinding chip is fastened at certain intervals. Furthermore, the grinding portion can be also configured by fastening a ring-shaped grinding chip to a ring-shaped supporting base plate. In this case, it is preferable to provide a plurality of grooves at certain positions of the grinding chip. In a case where grinding abrasive grain is used, the abrasive grain can be fastened directly to the supporting base plate by welding or electrodeposition. The grinding chips or the grinding abrasive grains are selected in consideration of price and the type of physical objects to be ground.

It is a further feature in configuration of the cup grinding wheel according to the present invention is that the grinding portion is formed of a plurality of small grinding portions (37, 47), with the small grinding portions being fastened at certain intervals to the rim fastening portion.

According to the invention, the small grinding portions are fastened to the rim fastening portion of the exterior base member with the screw members. Each of the plurality of small grinding portions is configured such that a grinding chip or grinding abrasive grain is fixed on one surface of a supporting base plate made of iron or steel, and the supporting base plate is fastened to the rim fastening portion. The invention can minimize the grinding surface made up of the grinding portion to achieve further weight reduction of the cup grinding wheel and to eliminate useless portions as much as possible. Furthermore, spaces between the small grinding portions can be used as air paths for cooling the grinding portion and ejecting paths which allow shavings to pass through. Each small grinding portion is formed of the supporting base plate and the grinding chip.

It is a still further feature in configuration of the cup grinding wheel according to the invention is that at least either the central base member or the exterior base member is made of resin material.

A resin material is lightweight, and has a property of decaying vibrations. Therefore, the present invention can realize the cup grinding wheel which is lightweight and reduces vibrations. Furthermore, the central base member and the exterior base member which make up the base plate portion of the cup grinding wheel can be made of materials having strength, weight, heat resistance and the like required according to positions where the central base member and the exterior base member are arranged. In a case where a material other than resin material is used for the central base member or the exterior base member, iron, steel, brass, aluminum or the like can be used.

It is another feature in configuration of the cup grinding wheel according to the invention is that the central base member is made of resin material, and the exterior base member is made of aluminum.

According to the invention, among components of the base plate portion of the cup grinding wheel, the central base member which is not required to have high-strength, compared with the other components, is made of a resin material which is lightweight, while the exterior base member which is required to have heat-resistance and high-strength in order to support the grinding portion which is heated during use is made of aluminum which is resistant to heat and lightweight for metal, and has high-strength. For example, the resin material is enforced plastic which has a melting point of 210 degrees Celsius or more, a continuous use temperature of 120 degrees Celsius or more, and a tensile strength of 75 MPa or more. It is preferable that the resin material is enforced plastic which has a melting point of 220 degrees Celsius or more, a continuous use temperature of 120 degrees Celsius or more, and a tensile strength of 100 MPa or more. It is more preferable that the resin material is enforced plastic which has a melting point of 260 degrees Celsius or more, a continuous use temperature of 155 degrees Celsius or more, and a tensile strength of 310 to 450 MPa or more. The resin material may be enforced plastic which has a melting point of 150 degrees Celsius or more, a continuous use temperature of 80 degrees Celsius or more, and a tensile strength of 60 MPa or more. Therefore, the present invention can realize the cup grinding wheel which is lightweight as a whole, and has properties necessary for the respective components of the base plate portion. During grinding of a physical object, furthermore, vibrations generated in the exterior base member are effectively decayed by the central base member made of the resin material. As a result, vibrations generated in the entire cup grinding wheel are significantly reduced, with noise also being significantly reduced. In the present invention, furthermore, aluminum can include aluminum alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a rotating tool having a cup grinding wheel according to the first embodiment of the present invention;

FIG. 2 indicates the cup grinding wheel, and more specifically, (a) is a front view, and (b) is a sectional view of (a) cut along a line 2-2;

FIG. 3 indicates a central base member, and more specifically, (a) is a front view, (b) is a sectional view of (a) cut along a line 3-3, and (c) is a back view;

FIG. 4 indicates an exterior base member, and more specifically, (a) is a front view, (b) is a sectional view of (a) cut along a line 4-4, and (c) is a back view;

FIG. 5 indicates a grinding portion, and more specifically, (a) is a front view, and (b) is a sectional view of (a) cut along a line 5-5;

FIG. 6 indicates a small grinding portion provided for a cup grinding wheel according to the second embodiment, and more specifically, (a) is a front view, and (b) is a sectional view of (a) cut along a line 6-6; and

FIG. 7 indicates a small grinding portion provided for a cup grinding wheel according to a modification of the second embodiment, and more specifically, (a) is a front view, and (b) is a sectional view of (a) cut along a line 7-7.

DESCRIPTION OF THE PREFERRED EMBODIMENT First Embodiment

The first embodiment of the present invention will now be described with reference to the drawings. FIG. 1 schematically indicates a rotating tool 10 having a cup grinding wheel 20 according to this embodiment. In explanations about the rotating tool 10 and the cup grinding wheel 20, the vertical direction and the front-rear direction are defined as indicated in FIG. 1. In FIG. 1, more specifically, the left is defined as the front, while the right is defined as the rear. The rotating tool 10 is a transportable tool held with user's hands to be operated. For example, the rotating tool 10 is used to grind a surface of a concrete wall of a structure to make the surface flat. The rotating tool 10 is configured such that a cup grinding wheel mounting portion 12 and a rotating shaft 13 protrude from an upper left part of a grip portion 11.

Furthermore, the cup grinding wheel 20 is mounted on the rotating tool 10 with flanges 14 a and 14 b, and a lock nut 14 c. Furthermore, a dust cover which is not shown is mounted on the grip portion 11 so that a part situated at the mounting portion 12 and the right side of the circumference of the cup grinding wheel 20 is covered with the dust cover. On the dust cover, furthermore, a dust bag is mounted with an outlet being interposed between the dust cover and the dust bag. Furthermore, the grip portion 11 is provided with various kinds of devices necessary for operation of the rotating tool 10 such as switches for activating and stopping the rotating tool 10, and indication lamps indicative of operational states.

The cup grinding wheel 20 is configured as indicated in FIGS. 2( a) and (b) such that a grinding portion 27 is fastened to a base plate portion 21 with screw members 29 each formed of a screw. More specifically, the base plate portion 21 is formed of a central base member 21 a and an exterior base member 21 b. The central base member 21 a is molded of reinforced plastic. As indicated in FIGS. 3( a) to (c), more specifically, the central base member 21 a is formed of a mounted portion 22 situated at the center, an inclined portion 23 which extends outward from the rim of the mounted portion 22 to be inclined diagonally frontward, and an engaging portion 24 extending outward from the rim of the inclined portion 23 to be in parallel with the mounted portion 22.

At the center of the mounted portion 22, a mounting hole 22 a penetrating in the front-rear direction is provided so that the rotating shaft 13 of the rotating tool 10 can be inserted through the mounting hole 22 a, while twelve ejecting holes 23 a penetrating in the front-rear direction are provided at regular intervals on the inclined portion 23. On the circumferential surface of the engaging portion 24, furthermore, an external thread 24 a is provided. As the enforced plastic for the central base member 21 a, a plastic having a melting point of about 210 degrees Celsius, a continuous use temperature (UL746B) of about 120 degrees Celsius, and a tensile strength of about 75 MPa is used. However, it is preferable to use an enforced plastic having greater values.

For example, it is more preferable that the enforced plastic has a melting point of 220 degrees Celsius or more, a continuous use temperature of 120 degrees Celsius or more, and a tensile strength of about 100 MPa or more. It is even more preferable that the enforced plastic has a melting point of 260 degrees Celsius or more, a continuous use temperature of 155 degrees Celsius or more, and a tensile strength of 310 to 450 MPa. As the enforced plastic for the central base member 21 a, furthermore, a plastic having values which are smaller than the above-described values can be used. For example, any enforced plastics can be used as long as the enforced plastics have a melting point of 150 degrees Celsius or more, a continuous use temperature of 80 degrees Celsius or more, and a tensile strength of 60 MPa or more.

The exterior base member 21 b is molded of aluminum, and is formed of a rim engaging portion 25 shaped like a tube (ring) which is short in an axial direction which is the front-rear direction, and a rim fastening portion 26 which is shaped like a flange plate and extends inwardly from the front edge of the rim engaging portion 25 as indicated in FIGS. 4( a) to (c). On the inner circumferential surface of the rim engaging portion 25, an internal thread 25 a which can be engaged with the external thread 24 a of the central base member 21 a is provided, so that the exterior base member 21 b is mounted on the central base member 21 a by engaging the internal thread 25 a with the external thread 24 a. A window hole 26 a provided at the center of the rim fastening portion 26 is designed to have a size which allows the mounted portion 22, the inclined portion 23 and an inner circumferential portion of the engaging portion 24 on front of the central base member 21 a to become exposed frontward when the exterior base member 21 b is mounted on the central base member 21 a. The rim fastening portion 26 is designed to have an external diameter of 96 mm and an internal diameter of 70 mm.

The grinding portion 27 is configured as indicated in FIGS. 5( a) and (b) such that six grinding chips 28 are fastened to the front of a supporting base plate 27 a formed of an iron plate shaped like a ring so that the grinding chips 28 are located at regular intervals along the ring. The supporting base plate 27 a has the same external diameter as the rim fastening portion 26, has an internal diameter of 77 mm and a thickness of 1.0 mm, and has six thread insertion holes 27 b provided at the center in the direction of width orthogonal to the circle so that the thread insertion holes 27 b are located at regular intervals along the circle. Each of the grinding chips 28 is configured by a sintered body obtained by sintering mixture of diamond abrasive grain and metal powder. Furthermore, each grinding chip 28 is designed such that when seen from the front, the grinding chip 28 is shaped like an arc obtained by dividing the front of the supporting base plate 27 a into approximately twelve equal parts in the circumferential direction. Each grinding chip 28 is designed to have a thickness of 5.0 mm.

The grinding chips 28 configured as above are arranged between the thread insertion holes 27 b on the front of the supporting base plate 27 a so that the outer circumferential part of each arc slightly juts out of the outer circumference of the supporting base plate 27 a. The six grinding chips 28 are fastened to the supporting base plate 27 a by brazing with silver to form the grinding portion 27. The grinding portion 27 is fastened to the exterior base member 21 b by inserting the screw members 29 into the thread insertion holes 27 b of the supporting base plate 27 a from the front to engage the end portion of each screw member 29 with the rim fastening portion 26. Furthermore, the cup grinding wheel 20 configured as described above is mounted on the rotating tool 10 through the flanges 14 a and 14 b, and the lock nut 14 c by inserting the rotating shaft 13 of the rotating tool 10 into the mounting hole 22 a.

For grinding of surface of a concrete wall of a structure to make the surface flat by use of the rotating tool 10 configured as above, a user activates the rotating tool 10 to rotate the cup grinding wheel 20, holding the grip portion 11 with a user's hand. Furthermore, pressing the front of the rotating cup grinding wheel 20 against the concrete wall by the user's manipulation with the user's hand, the user moves the rotating tool 10 along the surface of the concrete wall to make the surface of the concrete wall flat. During the manipulation, the grinding chips 28 of the grinding portion 27 are strongly in contact with the concrete wall to cause fierce vibrations on the grinding portion 27.

The vibrations are transferred from the grinding portion 27 to the base plate portion 21. Since the base plate portion 21 is configured by two members of the central base member 21 a and the exterior base member 21 b, the vibrations are hard to be transferred between the exterior base member 21 b and the central base member 21 a. Furthermore, the central base member 21 a has a property of decaying vibrations. Therefore, vibrations conveyed to the user are suppressed to enhance ease of use. Furthermore, the reduced vibrations can also reduce noise to prevent the user from being annoyed and to prevent loud noise from occurring around the worksite.

In this case, the configuration that the exterior base member 21 b is made of aluminum while the central base member 21 a is provided with the ejecting holes 23 a can also suppress vibration and noise. Furthermore, part of concrete shavings shaved away from the concrete wall by the grinding portion 27 is ejected to the outside from gaps between the grinding chips 28, while the other concrete shavings pass through the ejecting holes 23 a to enter the dust cover to be ejected from an outlet of the dust cover to be housed in the dust bag.

As described above, the cup grinding wheel 20 according to this embodiment is configured such that the base plate portion 21 is formed of the separately provided central base member 21 a and exterior base member 21 b. Therefore, even though vibrations are transferred from the grinding portion 27 to the exterior base member 21 b during grinding, the central base member 21 a serves as a vibration isolator or an interrupter which interrupts transmission of vibrations to reduce vibrations of the exterior base member 21 b or to interrupt vibrations of the exterior base member 21 b to prevent the vibrations from growing. As a result, vibrations of the entire cup grinding wheel 20 are significantly reduced, so that noise can be significantly reduced.

Furthermore, the grinding portion 27 is fastened to the rim fastening portion 26 of the exterior base member 21 b by the screw members 29. Therefore, even if the grinding portion 27 is heated to a high temperature by friction heat caused by grinding, the grinding portion 27 will not be released from the exterior base member 21 b due to the heat generated on grinding portion 27. The same holds true for a case where the grinding portion 27 wears away to allow heat generated on the grinding portion 27 to be easily transferred to the neighboring exterior base member 21 b, so that not only the grinding portion 27 but also the exterior base member 21 b are heated to a high temperature. In addition, fastening of the grinding portion 27 to the exterior base member 21 b is facilitated, so that the manufacture of the cup grinding wheel 20 is also facilitated, with the exchange of the exhausted grinding portion 27 being also facilitated.

Furthermore, since the central base member 21 a is made of reinforced plastic, with the plurality of ejecting holes 23 a being provided around the mounting hole 22 a, the weight reduction of the cup grinding wheel 20 can be achieved. In addition, prevention of vibration and noise reduction can be effectively achieved. Furthermore, the exterior base member 21 b is made of aluminum. Therefore, the weight reduction of the cup grinding wheel 20 as a whole can be achieved, while the exterior base member 21 b which supports the grinding portion 27 has high strength and heat resistance. Furthermore, since the grinding portion 27 is formed of the supporting base plate 27 a and the grinding chips 28, with the supporting base plate 27 a being fastened to the rim fastening portion 26 of the exterior base member 21 b, holes for inserting the screw members 29 are unnecessary for the grinding chips 28, which facilitates molding of the grinding chips and eliminates useless space which cannot be used for grinding from the grinding chips 28.

Second Embodiment

FIGS. 6( a) and (b) indicate a small grinding portion 37 provided on a cup grinding wheel according to the second embodiment of the present invention. The cup grinding wheel has six small grinding portions 37 to form a grinding portion. Each small grinding portion 37 is configured such that the grinding chip 28 is fastened to the front of a small supporting base plate 37 a formed of an iron plate. The small supporting base plate 37 a is shaped like an arc obtained by dividing the above-described supporting base plate 27 a into six equal parts in the circumferential direction, with both end portions of the small supporting base plate 37 a having a thread insertion hole 37 b. The grinding chip 28 is fastened by brazing to be situated between the thread insertion holes 37 b on the front of the small supporting base plate 37 a so that the outer circumferential part of the arc slightly juts out of the outer circumference of the small supporting base plate 37 a.

Except the grinding portion formed of the six small grinding portions 37, the cup grinding wheel of this embodiment is designed the same as the above-described cup grinding wheel 20. Each small grinding portion 37 is fastened to the exterior base member 21 b by inserting the screw members 29 into the thread insertion holes 37 b of the small supporting base plate 37 a from the front to engage the top portion of each screw member 29 with the rim fastening portion 26. As a result, the six small grinding portions 37 are connected with each other to be shaped like a ring.

Because of this configuration, even if a part of the six small grinding portions 37 is broken, only the broken small grinding portion 37 can be replaced. The other operational advantages of the second embodiment are the same as those of the first embodiment. In this embodiment, furthermore, the small supporting base plate 37 a may be designed to be smaller than the one obtained by dividing the supporting base plate 27 a into six equal parts in the circumferential direction. This modification can minimize the grinding surfaces of the grinding portion to achieve further weight reduction of the cup grinding wheel and to eliminate useless parts as much as possible.

(Modification of Second Embodiment)

FIGS. 7( a) and (b) indicate a small grinding portion 47 provided on a cup grinding wheel according to a modification of the second embodiment of the present invention. The cup grinding wheel has three small grinding portions 47 to form a grinding portion. Each small grinding portion 47 is configured such that three grinding chips 28 are fastened to the front of a small supporting base plate 47 a formed of an iron plate. The small supporting base plate 47 a is formed of an arc-shaped portion 47 b shaped like an arc which is slightly shorter than an arc obtained by dividing the above-described supporting base plate 27 a into three equal parts in the circumferential direction, and a short projecting portion 47 c which bends and extends inwardly from the center of an inner circumferential portion of the arc-shaped portion 47 b. Furthermore, five thread insertion holes 47 d are scattered at both sides of the inner circumference of the arc-shaped portion 47 b and at the central portions of the outer circumference of the arc-shaped portion 47 b, and at the end of the projecting portion 47 c.

Two of the three grinding chips 28 are fastened by brazing to the both sides of the front of the arc-shaped portion 47 b so that the two grinding chips 28 are located where no thread insertion hole 47 d is provided, with respective arc-shaped outer circumferential parts of the two grinding chips 28 slightly juts out of the outer circumference of the arc-shaped portion 47 b. The rest of the grinding chips 28 is fastened by brazing to a part where no thread insertion hole 47 d is provided so that the grinding chip 28 is arranged along a center portion of the projecting portion 47 c such that one end of the grinding chip 28 slightly juts out of the center of the outer circumference of the arc-shaped portion 47 b. The grinding chip 28 fastened to the projecting portion 47 c is arranged so that, when the cup grinding wheel rotates for grinding, the outer part of the arc of the grinding chip 28 faces in the direction in which the cup grinding wheel 20 travels.

Except the grinding portion formed of the three small grinding portions 47, the cup grinding wheel of this modification is designed the same as the above-described cup grinding wheel 20. Each small grinding portion 47 is fastened to the exterior base member 21 b by inserting the screw members 29 into the thread insertion holes 47 d of the small supporting base plate 47 a from the front to engage respective top portions of the screw members 29 with the rim fastening portion 26. As a result, the outer circumferential parts of the three small grinding portions 47 are arranged to be shaped like a ring having intervals. The modification configured as above can have wide grinding surfaces to enable efficient grinding of a physical object. The other operational advantages of the modification are the same as those of the second embodiment.

The cup grinding wheel according to the present invention is not limited to the above-described embodiments and modification, but can be variously modified. For instance, although the above-described embodiments and modification are configured such that the central base member 21 a is made of reinforced plastic, with the exterior base member 21 b being made of aluminum, materials for the central base member 21 a and the exterior base member 21 b are not limited to the materials described above but can be changed to different materials as desired. Materials available for the central base member 21 a and the exterior base member 21 b include resin materials other than the above-described reinforced plastic, iron, steel and brass, in addition to aluminum. More specifically, any two of those materials may be used. Alternatively, one of those materials may be used both for the central base member 21 a and the exterior base member 21 b. In a case where different materials are used, either of the materials can be used for either the central base member 21 a or the exterior base member 21 b. Further, the supporting base plate 27 a and the small supporting base plates 37 a, 47 a of the above-described embodiments and modification are formed of a steel plate instead of the iron plate.

Furthermore, although the above-described embodiments and modification are configured such that the grinding portions 27, 37 and 47 have the supporting base plate 27 a, the small supporting base plates 37 a and the small supporting base plates 47 a, respectively, the supporting base plate 27 a, the small supporting base plates 37 a and the small supporting base plates 47 a may be eliminated. In this case, each grinding chip 28 is provided with thread insertion holes so that the thread insertion holes are used to fasten the grinding chip 28 to the rim fastening portion 26 with the screw members 29. As the grinding chips 28, furthermore, not only the sintered products using diamond abrasive grain but also any abrasive grains can be used as long as the abrasive grains are available for grinding. Instead of the grinding chips 28, furthermore, a ring-shaped grinding chip having a plurality of grooves at certain positions may be used.

In the above-described embodiments and modification, furthermore, the grinding portions 27 and the like are configured by welding the grinding chips 28 to the supporting base plates 27 a, and the like. Instead of the grinding chips 28, however, grinding abrasive grains may be used. In this case, the grinding abrasive grains are fastened to the supporting base plates 27 a and the like by welding or electrodeposition. Furthermore, the grinding portions 27 and the like may employ not only the above-described grinding chips 28 and abrasive grains but also anything as long as it is available for grinding.

Furthermore, although the above-described embodiments and modification are configured such that the central base member 21 a has the ejecting holes 23 a, the ejecting holes 23 a may not be provided. Furthermore, the central base member 21 a and the exterior base member 21 b may not be shaped as indicated as above, but may be shaped differently. More specifically, the central base member 21 a may be shaped like a circular disc having a peripheral wall on an outer circumference of the circular disc, or may have a shape which is almost flat. The exterior base member 21 b may have any shape as long as the exterior base member 21 b has an outer circumference engaging portion which can be engaged with the outer circumferential portion of the central base member 21 a, and a flanged rim fastening portion having a front surface to which the grinding portion 27 or the like can be fixed. Furthermore, the engagement between the central base member 21 a and the exterior base member 21 b is not limited to the thread engagement, but may be done by different engagement such as engagement between groove extending in the front-rear direction (in a direction in which the central base member 21 a and the exterior base member 21 b have a thickness) and protrusion. Furthermore, a physical object to be ground by the cup grinding wheel according to the present invention is not limited to concrete walls but may be anything as long as they need to be ground to make them flat. 

What is claimed is:
 1. A cup grinding wheel which is mounted on a rotating tool and rotates to grind a surface of a physical object, the cup grinding wheel comprising: a central base member approximately shaped like a circular disc having a mounting hole at a center thereof; an exterior base member formed of a rim engaging portion which can be engaged with a circumferential portion of the central base member, and a flanged rim fastening portion provided on one rim of the rim engaging portion; a grinding portion; and a screw member for fastening the grinding portion to one surface of the rim fastening portion.
 2. The cup grinding wheel according to claim 1, wherein a plurality of ejecting holes are provided around the mounting hole of the central base member.
 3. The cup grinding wheel according to claim 1, wherein the grinding portion is configured such that a grinding chip is fixed on one surface of a supporting base plate made of iron or steel, and the supporting base plate is fastened to the rim fastening portion.
 4. The cup grinding wheel according to claim 3, wherein the grinding chip is a sintered product obtained by molding and sintering diamond abrasive grain by use of bonding agent.
 5. The cup grinding wheel according to claim 3, wherein the grinding chip is fastened to the supporting base plate by brazing.
 6. The cup grinding wheel according to claim 3, wherein the grinding chip is formed of a plurality of grinding chip, the supporting base plate is ring-shaped, and the plurality of grinding chips are fixed at a certain interval on the supporting base plate.
 7. The cup grinding wheel according to claim 3, wherein the supporting base plate and the grinding chip are ring-shaped respectively, and a plurality of grooves are provided at certain positions of the grinding chip.
 8. The cup grinding wheel according to claim 1, wherein the grinding portion is configured such that grinding abrasive grain is fixed on one surface of a supporting base plate made of iron or steel, and the supporting base plate is fastened to the rim fastening portion.
 9. The cup grinding wheel according to claim 8, wherein the abrasive grain is fastened directly to the supporting base plate by welding or electrodeposition.
 10. The cup grinding wheel according to claim 1, wherein the grinding portion is formed of a plurality of small grinding portions, with the small grinding portions being fixed at certain intervals to the rim fastening portion.
 11. The cup grinding wheel according to claim 10, wherein each of the plurality of small grinding portions is configured such that a grinding chip or grinding abrasive grain is fixed on one surface of a supporting base plate made of iron or steel, and the supporting base plate is fastened to the rim fastening portion.
 12. The cup grinding wheel according to claim 10, wherein the plurality of small grinding portions are fastened to the rim fastening portion with screw members.
 13. The cup grinding wheel according to claim 1, wherein at least either the central base member or the exterior base member is made of resin material.
 14. The cup grinding wheel according to claim 1, wherein the central base member is made of resin material, and the exterior base member is made of aluminum.
 15. The cup grinding wheel according to claim 14, wherein the resin material is enforced plastic which has a melting point of 150 degrees Celsius or more, a continuous use temperature of 80 degrees Celsius or more, and a tensile strength of 60 MPa or more. 